fix for incorrect opencl device selection
added 3-channels support to morphology operations
update error message
eliminated restriction src[i].channels() == 1 in cv::merge
fixed bug in cv::ocl::predictOptimalVectorWidth
Merge pull request #2492 from KonstantinMatskevich:ocl_fix_wrongdevice
Merge pull request #2494 from ilya-lavrenov:tapi_merge
Merge pull request #2496 from ilya-lavrenov:tapi_imgproc